Swift: UIViewController 实例未从 URLSession 委托方法接收更新

Question

我无法从 URLSession Delegate 方法更新 UIViewController。

我有一个包含不同用户的 TableView 和一个包含与该用户关联的多个图像的详细视图控制器。

现在我需要下载图像，我创建了一个背景 URL session，每个用户都有一个唯一标识符。我这样做是因为我必须为 TableView 中的每个用户使用相同的 Detail UIViewController。

>DetailViewController

// Download service for this controller. Shared instance because it tracks active downloads. /// [URL:Download]

let downloadService:DownloadService = MediaDownloadManager.shared.downloadService


// A Unique background download session for each user

lazy var downloadSession: URLSession = {
    let identifer:String = "com.example.app" + "-user-" + userID   /// userID passed from parent
    let configuration = URLSessionConfiguration.background(withIdentifier: identifer)
    return URLSession(configuration: configuration, delegate: self, delegateQueue: nil)
}()


override func viewDidLoad() {
    super.viewDidLoad()
    downloadService.downloadsSession = downloadSession

    /// I get this every time when I exit and re-enter the controller which is obvious.
    /// [DEBUG CONSOLE] A background URLSession with identifier com.example.app-user-12 already exists!
}

并且我已将 URLSession Delegates 纳入我的 DetailViewController 以跟踪下载进度和更新视图。

extension DetailViewController: URLSessionDelegate, URLSessionDownloadDelegate {
    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) { /// }
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) { /// }
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) { /// }
}

现在的问题是，当我运行应用程序并为用户打开 DetailViewController 并开始下载我需要的图像时，它工作正常并且我从 urlsession 委托方法，然后更新我的 tableView。

但是当我滑回 TableViewController 然后为同一用户重新打开 DetailViewController 并开始下载图像时，DetailViewController 的 tableView 不会从那些 urlsession 委托方法接收更新。

I found what's exactly causing this problem. The URLSession Delegate methods capture the instance of my view controller. Which means when I update my DetailViewController's TableView, from the URLSession Delegate methods, they actually try to update the tableView of the first and former UIViewController instance.

下面是更新单元格进度的代码。

func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
    guard let downloadURL = downloadTask.originalRequest?.url else {
        printAndLog(message: "### \(#function) Failed to retrieve original request download url", log: .network, logType: .error)
        return
    }
    let progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
    let downloadedSize = ByteCountFormatter.string(fromByteCount: totalBytesWritten, countStyle: .file)
    let totalSize = ByteCountFormatter.string(fromByteCount: totalBytesExpectedToWrite, countStyle: .file)

    print(self) /// Same instance everytime.

    if let download = downloadService.activeDownloads[downloadURL] {
        download.progress = progress
        DispatchQueue.main.async {
            if let index = self.images.firstIndex(of: download.image),
                let cell = self.tableView.cellForRow(at: IndexPath(row: index, section: 0)) as? ImageCell {
                cell.updateProgress(progress: download.progress, loadedSize: downloadedSize, totalSize: totalSize)
            }
        }
    }
}

How can I solve this annoying problem? I know singletons are a good answer but I want to avoid singletons as much possible.

我无法使用 downloadSession.finishTasksAndInvalidate()，因为我认为当我退出控制器时它不会继续我的下载。

我的要求：

1-) 当我退出控制器时，下载应该会继续。

2-) DetailViewController 的每个实例都应该可以下载，即每个用户

3-) 后台下载当然是必须的。

如果您认为此问题的标题有误，请见谅。

Answer 1

您需要添加另一个抽象级别。视图不应直接订阅后台下载进程。其他一些 class 应该订阅它，并且应该从其他 class.

更新视图

这样您只有一次 class 订阅，即使多个视图需要数据也是如此。

Swift: UIViewController 实例未从 URLSession 委托方法接收更新

Swift: UIViewController instance not receiving updates from URLSession Delegate methods

ios

nsurlsession

swift